The robotic applications developed and exploited until today almost exclusively use electromechanical actuators. Even though the technology of electromechanical devices is very well established and has thorough theoretical background, control methods and reliable applications demonstrated during several decades, it is reaching its limits. Such devices comprise rigid links to connect rotating joints, gears and bearings and are, therefore, unavoidably complex, rigid and noisy. At the current state, reducing the size and energy consumption of such devices is extremely challenging task.
An alternative approach is to use shape-changing materials, such as electroactive polymers (EAP), to actuate robotic devices.
Electroactive properties of ionic polymers are caused by mobility or diffusion of ions. EAP materials of this group include carbon nanotubes, conductive polymers, ionic polymer gels and ionic polymer metal composites. Ionic polymers bend when electric stimulation is applied. They produce large displacement when stimulated and operate at low voltages. Because of the dynamic processes inside the materials they do not keep the strain but relax after a while to the initial configuration. Therefore the applications of such materials are usually inspired by aquatic animals, e.g., mimicking motion of a caudal fin, pectoral fins, a mollusk or a tadpole.
FIG. 1 depicts IPMC material sheet in a bent configuration with the opposite driving voltage polarity (A and C) and an initial configuration with no electric stimulus applied (B). Ionic polymer materials are made of a highly porous ion fluorinated polymer, like Nafion®, Flemion®, Teflon® and their modifications, filled with ionic conductive liquid. During material fabrication the proton connected to the terminal group (the chemical unit in the end of a polymer chain), is replaced with a metal ionic cat-ion (Na+, Li+). These cat-ions will dissociate in water, so that terminal groups will have a negative charge and at the same time there will be an excess of free cat-ions in the material (see FIG. 2, to the left). A sheet from this kind of material is then covered with a metal coating, usually platinum or gold.
Since water molecules are dipoles, they orient themselves in electromagnetic field and get attached to the free metal cat-ions. An applied electric field causes an electric current and the cat-ions start to move to one side of the material causing expansion of the material on that side and contraction on the other side (FIG. 2, in the middle).
The bent conformation is an imbalanced situation. Water starts to diffuse in the opposite direction and the polymer sheet relaxes after some time (FIG. 2, to the right). These materials do not keep their position under direct current. At the same time, their action length is remarkable and they operate at low voltage (1.2-7V). The actuator performance of IPMCs depends on their morphology, as well as on other parameters such as membrane thickness, electrodes surface conductivity, solvent type and anion doping. These parameters can be tuned during the manufacturing process. IPMC is therefore an engineering material that can be customized to application requirements.
In addition to actuation properties, IPMC materials can also work as sensors. If the IPMC material is mechanically bent, a voltage is generated between the surface electrodes due to the non-uniform concentration of ions in the membrane. The effect is observed when the sheet is in motion, i.e., the sensor works as an accelerometer. For that reason, IPMC sensors have been investigated as vibration sensors for active noise damping.
However, the signal of the sensor is very week (1 mV-2 mV) while the actuator is at the same time driven with 2 V-4 V input signals. The equivalent circuit in FIG. 3 shows that the IPMC material is essentially an infinite lossy transmission line. Therefore, the signals, traveling back and forth along the material are considerably distorted and delayed. It is difficult to distinguish the sensor signal from the distorted and delayed driving signal.
An alternative way, described in PCT/EE2007/000005 (authors M. Kruusmaa, A. Punning, A. Aabloo), is to use the change of the surface resistance to measure the bending of the actuator. The resistance of the metal surface electrodes of the IPMC sheet (shown as Ra and Rb in FIG. 3) changes during bending and the change of the resistance is highly correlated to the bending curvature. The change of the surface resistance is not caused by the electroactive properties of the IPMC sheet (like in the case of vibration sensors) but by the properties of the metal surface electrode. The resistance of a thin metal coating increases or decreases if the metal layer is compressed or stretched out. This effect can be used to determine the position of the IPMC sheet and a design has been is proposed that permits the IPMC sheet to be used as a self-sensing actuator. The output signal of such a sensor is at least an order of magnitude stronger (10 mV-20 mV) than of the conventional vibration sensor with a very good signal to noise ratio. The sensor signal is at least an order of magnitude stronger and is not distorted by the dynamics of the transmission line. Unlike the conventional vibration sensor this self-sensing actuator gives accurate information about the configuration of the sheet also when the sheet is not in motion. Therefore it can be used as a position sensor but also as an accelerometer if the sensor data is sampled over time.
Compared to electromechanical devices, EAPs have several complimentary advantages. They are lightweight, soft and flexible, easy to miniaturize, and permit distributed actuation and sensing. The behavior of the EAP materials in electric field somewhat resembles the performance of biological muscles, therefore EAP materials are considered suitable for biomimetic devices. Although compared to electromechanical devices, EAPs have low output force or small strain (depending on the material used), high energy consumption and lack of well-established control methods, they are a promising alternative to overcome the drawbacks of bulky, noisy, rigid electromechanical devices.
Short link comprising IPMC material acts similarly to a rotational joint as described in Estonian patent application No EE200700028, filed on 7 Jun. 2007, inventors Kruusmaa et al, applicant Tartu University.
Thus, there is a need for new robotic applications with actuators, based on shape changing materials like ionic polymer metal composite materials.